Title:
Effect of Reinforcement Layers, Side Cover, and Bond Stress on Curved-Bar Nodes
Author(s):
Hwa-Ching Wang, Christopher S. Williams, and Gary J. Klein
Publication:
Structural Journal
Volume:
119
Issue:
3
Appears on pages(s):
277-290
Keywords:
bend radius; curved-bar node; design code; frame corners; knee joints; strut-and-tie method
DOI:
10.14359/51734490
Date:
5/1/2022
Abstract:
A curved-bar node is formed in a strut-and-tie model at locations where two ties representing the bend region of continuous reinforcing bars are equilibrated by a strut or the resultant of multiple struts. The most common example is present within a strut-and-tie model of a knee joint subjected to closing moments. In ACI 318-19, design provisions for curved-bar nodes were introduced based on expressions derived by considering equilibrium of the assumed nodal region and verified with a limited number of experimental studies available at the time. It was recognized, however, that additional investigations were needed to understand the applicability of these expressions for various cases. An experimental program consisting of tests on 20 knee joint specimens was conducted to study the influence of multiple layers of longitudinal reinforcement, clear side cover, and bond stresses within the bend region of the longitudinal bars. The results demonstrate that curved-bar nodes with two layers of longitudinal reinforcement can be designed following the expression for the minimum bend radius of reinforcing bars in Section 23.10.2 of ACI 318-19. Furthermore, the results indicate that no changes to the current provisions regarding clear side cover are needed. However, the code provision related to the development of bond stresses appears to be unnecessary based on the tests.
Related References:
1. ACI Committee 318, “Building Code Requirements for Structural Concrete (ACI 318-19) and Commentary (ACI 318R-19),” American Concrete Institute, Farmington Hills, MI, 2019, 624 pp.
2. Klein, G. J., “Curved-Bar Nodes: A Detailing Tool for Strut-and-Tie Models,” Concrete International, V. 30, No. 9, Sept. 2008, pp. 42-47.
3. FIP Commission 3, “Practical Design of Structural Concrete,” FIP Recommendations 1996, Fédération Internationale de la Precontrainte, Brussels, Belgium, Sept. 1999, 113 pp.
4. Stroband, J., and Kolpa, J. J., “The Behaviour of Reinforced Concrete Column-to-Beam Joints. Part 1. Corner Joints Subjected to Negative Moments,” Delft University of Technology, Department of Civil Engineering, Delft, the Netherlands, 1983, 105 pp.
5. Luo, Y. H.; Durrani, A. J.; Bai, S.; and Yuan, J., “Study of Reinforcing Detail of Tension Bars in Frame Corner Connections,” ACI Structural Journal, V. 91, No. 4, July-Aug. 1994, pp. 486-496.
6. Bai, S., and Luo, Y. H., “Concrete Frame Corners,” Pacific Concrete Conference, New Zealand, Nov. 1988, pp. 157-168.
7. Hotta, H., and Nishizawa, N., “Experimental Study on Influence of Arrangement of Main Bars on Stress Transmission in Reinforced Concrete Beam-Column Knee Joints,” Lisboa, Portugal, 2012, 10 pp.
8. Wang, H.-C.; Williams, C. S.; and Klein, G. J., “Effect of Bend Radius of Reinforcing Bars on Knee Joints under Closing Moments,” ACI Structural Journal, V. 117, No. 5, Sept. 2020, pp. 315-326.
9. Wang, H.-C., “Behavior and Design of Concrete Frame Corners: Strut-and-Tie Approach,” PhD dissertation, Purdue University, West Lafayette, IN, Dec. 2020, 336 pp.
10. ASTM A615/A615M-16, “Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement,” ASTM International, West Conshohocken, PA, 2016, 6 pp.
11. ACI Committee 374, “Guide for Testing Reinforced Concrete Structural Elements Under Slowly Applied Simulated Seismic Loads (ACI 374.2R-13),” American Concrete Institute, Farmington Hills, MI, 2013, 19 pp.